Local co-moving reference frame of the emitting plasma. The direction of the local B field and the local normal to the emitting (column) surface are denoted by e_B and e_S, respectively, where e_B ⋅ e_S = 0. The local emission direction of the photon is denoted by k′. The photon emerges from the plane of the emitting surface (gray) with an angle, ζ′, with respect to the surface normal, e_S. The angle, η′, is measured with respect to the B field, with μ′ = cos η′ = k′ ⋅ e_B, while η_S = η′_S is the angle between the surface normal, e_S, and the projection of k′ into the plane perpendicular to the B field, with μ_S = cos η_S. The emission pattern, i.e., the angle dependence of the specific intensity in Eq. (1), projected into the three planes of the reference frame, is shown in orange. The maximum emission is reached for ζ′ = 0° or equivalently η′ = 90° and η′_S = 0°.